Compact multibeam laser light source and interleaving raster scan line method for exposing printing plates

ABSTRACT

An individually drivable array of single stripe laser diodes is proposed for imaging printing plates. An imaging optics is used to produce n image points which have a spatial interval l between adjacent points. An interleaving raster scan line method is indicated, which, given proper selection of the increment, enables each dot to be exposed exactly once.

BACKGROUND INFORMATION

The present invention is directed to a device for imaging printingplates using an array of n laser diodes.

For some time now, devices and methods have been known, which make itpossible to image a printing plate, whether it be a flat or curvedsurface, through exposure to laser radiation. Devices and methods ofthis kind are used, in particular, in so-called CtP systems,computer-to-plate, or direct imaging print units or printing presses formanufacturing offset printing forms.

At the present time, printing plates are primarily imaged by laser diodesystems. Their inherent system properties prevent them from reaching thephysical limits of the beam quality. In particular, their low beamquality limits their depth of focus, so that an autofocusing system isneeded at high resolutions. Two different approaches are currently usedfor multibeam imaging, i.e., for simultaneously exposing a plurality ofimage points on various media, such as printing plates, films, datacarriers or the like. On the one hand, the radiation from individuallaser diodes or an array of laser diodes can be directly applied viaoptical elements, such as lenses, mirrors or fibers, to the medium to beimaged. On the other hand, the radiation from a laser light source,typically laser diode bars, can be projected via diverse opticalelements onto an array of n modulators. For the most part, these areelectrooptical or acoustooptic modulators. By selectively driving the nmodulators, one can select individual beams from the entire radiationand modulate their power. The selected, power-modulated beams aresupplied via further optical elements to the medium to be imaged.

European Patent Application No. 0 878 773 A2 describes an optical systemfor imaging an array of light sources, in particular an individuallyaddressable array of laser substantially greater than their emitterheight. The emission region is typically about 1 micrometer high and 60micrometers wide. The optical system is composed of a system ofnon-anamorphotic imaging lenses and of a cylinder lens, which is placedbetween the array and the imaging lens system and images the laserradiation onto the scanning surface. This surface usually does not liein the foci of the laser beams, so that a widening of the shortdimensions of the imaged emission surface occurs.

U.S. Pat. No. 5,521,748 describes a system for exposing image data usingan individual laser or an array of diodes and a light modulator. Thelight transmitted by the laser or the array is imaged onto a modulatorhaving a row of light-modulating elements of the reflectance ortransmittance type. Once selection and power modulation are carried out,the radiation is imaged onto a surface having light-sensitive material,forming individual image points. To place image points of this kind on acomplete, two-dimensional surface, a relative motion of the image pointsto the light-sensitive material is provided. In the interplay resultingfrom generation of the individual points and the relative motion, thedesired image data are then exposed on the two-dimensional surface. Therelative motion between the light beams emanating from the lightmodulator and the light-sensitive material can be effected on acylindrical configuration such that lines are exposed in a meander shapealong the axis of symmetry of the cylinder, or such that lines runaround the cylinder in a helical form.

U.S. Pat. No. 5,691,759 discusses a multi-beam laser light source, whichproduces raster scan lines on a medium using the so-called interleavingraster scan line method. The interleaving raster scan line method isdistinguished by the following properties. A laser light source emitsradiation, from which n image points are produced using modulated powerby employing suitable imaging optics and modulation. These n imagepoints are arranged in a row, and the distance between two adjacentpoints is (n+1)p, p being the distance between the dots. Provision ismade between the medium and the image points for a relative motion inboth directions, spanning the surface of the medium. Once n points areimaged, the medium is displaced relatively to the image points with atranslational component that is perpendicular to the direction definedby the axis of the image points, so that n points can again be exposedat another location of the medium. In this manner, so-called scan linesof image points are formed, initially at a distance of (n+1)p, which areproduced by laser radiation, whose power is modulated in dependence uponthe image information. Upon completion of a scan having a translationalcomponent in the perpendicular direction, a displacement by the distance(n×p) follows in parallel to the direction defined by the axis of the nimage points. The n image points are then shifted again with atranslational component that is perpendicular to the direction definedby the axis of the image pixels on the surface, forming further scanlines. Thus, each raster scan line is separated from its immediateneighbor by the pitch distance p between the dots. Using a plurality ofoptical beams from a laser light source, an overlapping of the scanlines ensues (interleaving raster scan line method).

An enhanced interleaving raster scan line method for a multibeam laserlight source is described in European Patent Application No. 0 947 950A2. In the case of n image points having a pitch distance p of the dots,each of whose adjacent image points are separated by the distance(q×n+1)p, q being a natural number, an incremental distance of n×presults by which the medium must be moved between the marking of twoscan lines. An overlapping (interleaving) of the scan lines is therebyachieved, in other words, the new scan lines are written between the oldscan lines. By properly selecting the displacement in parallel to theaxis defined by the image points, by the distance n×p, an imaging isthen possible, without one location, where image information is to bescanned, being repeatedly exposed to one image point of a laser. Whatdistinguishes the described method is that adjacent image points of thelaser diodes are spaced further apart, in each case, than the width ofthe displacement by which the medium is moved between the old and newscan lines.

Various disadvantages are associated with each of the known devices. Theradiation emitted by broad array laser diodes, laser diode bars, andlaser diode stacks exhibits a low beam quality, as quantified by thediffraction index M². Even with correction, the attainable depth offocus is only suited for imaging at a low resolution, typically 1,270dpi. Therefore, to produce very small dots, for example resolutions ofabout 2,540 dpi, an autofocusing system is necessary, which requires acomplex mechanical and electrical design. If the light source andmodulator are provided separately, there is an increased requirement foroptical, electronic and mechanical components, as well as forsubstantial overall space. Many components need to be adjusted, and theservice life can be clearly limited. The temperature management of thecomponents turns out to be just as problematic. Only a limited, minimalphysical size is possible when a device for imaging printing plates isassembled from discrete components. The described interleaving rasterscan line method is not suited for compact laser light sources, sincethe distance between adjacent image points must always be one unit pgreater than the number of beams, so that one must revert to scanningmethods in which image points are set densely together.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a device for imagingprinting plates using an array of n laser diodes, whose emitted lightexhibits a good beam quality and which renders possible a compactdesign. An additional or alternate object of the present invention is toprovide an improved interleaving raster scan line method.

The present invention provides a device for imaging printing platesusing an array (10) of n laser diodes which are imaged onto n imagepoints (110), so that one laser diode (12) is allocated to each i-thpoint having i from {1, . . . , n}, the n image points (110) beingseparated by a spatial interval of adjacent points l, and a pitchdistance p of the dots being provided. The laser diodes (12) areindividually drivable single stripe laser diodes.

The present invention also provides a method, i.e., a so-calledinterleaving raster scan line method, for imaging printing plates bygenerating raster points using an array of n laser light sources, whichare imaged using an imaging optics onto n image points, which arearranged on a line, the n image points being separated by a spatialinterval of adjacent points l, comprising the following method steps:

simultaneous generation of n image points on the printing plate by anumber of laser light sources;

generation of a relative motion between the image points and printingplate;

displacement of the image points with a translation componentperpendicular to the axis defined by the line of the image points by afirst specific amount;

displacement of the n image points in the direction defined by the nimage points by a second specific amount; and

iteration of the displacements in question, wherein the amount of thesecond specific displacement is greater than the spatial interval l ofadjacent image points.

In accordance with the present invention, the device for imagingprinting plates includes an array of n single stripe laser diodes. Eachsingle stripe laser diode can be driven individually. The n laser beamscan preferably be imaged onto the medium using light-transmitting means,such as lenses, mirrors, optical fibers or the like. The n image pointsproduced with the assistance of imaging optics are advantageouslydisposed on a line and have a spatial interval l between adjacentpoints. It is generally only necessary, however, that the n image pointsprojected onto a predefined line in the surface of the printing platehave a constant spatial interval l. A relative motion takes placebetween the medium and the image points in both directions, spanned bythe surface of the medium. In addition to the motion, which, in order todisplace the image points with a translational component perpendicularto the direction defined by the line of the n image points or by thepredefined line, on which the projected n image points exhibit aconstant spatial interval l, a displacement takes place in parallel tothe direction defined by line of the n image points or by the predefinedline, on which the projected n image points exhibit a constant spatialinterval l. The amount of this displacement is advantageously greaterthan or equal to the spatial interval l between the n image points.Raster scan lines are produced which exhibit a pitch distance p betweenthe dots, pitch distance p between the dots being smaller than spatialinterval l between the image points.

One preferred specific embodiment provides that the power supply to thearray of the laser diodes be regulated. A suitable detector elementadvantageously checks for proper functioning, and, as the case may be,for potential malfunctioning of a single stripe laser diode, either onthe outcoupling side of the laser diode or, however, at another cavitymirror. In this context, the detector element can be both a detectorrow, as well as an individual detector, which scans the individualsingle stripe laser diodes.

One derives a number of advantages from the use of an array of n singlestripe laser diodes, which can be individually driven, and from theapplication of the corresponding interleaving raster scan line method toimage printing plates. An excellent beam quality is achieved by usingsingle stripe laser diodes. Typically, the value of diffraction index M²is slightly higher than one. In a compact design, a high level ofintegration can be achieved: radiation source, modulation, and controlcan be combined in one component. The result is fewer optical componentsand, therefore, less need for adjustment of sensitive components. Theservice life of the component is essentially limited only by the servicelife of the laser. The compact, modular design makes the systemscalable. A high-performance stability is assured by a rapid control.The high level of integration provides for a simpler temperaturemanagement, since it is only necessary to cool this one component. Dueto the low diffraction index M², a maximum possible depth of focus isachieved when focusing.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages and beneficial specific embodiments of the presentinvention are presented on the basis of the following figures, as wellas their descriptions, in which:

FIG. 1 shows a schematic view of the typical geometry in the imaging ofa printing plate by an array of laser diodes having n laser beams;

FIG. 2 shows a schematic view of the imaging of a printing plate on acylinder by an array of n laser beams; and

FIG. 3 shows an example of imaging including an array of five imagepoints in the interleaving raster scan line method.

DETAILED DESCRIPTION

FIG. 1 depicts a typical geometry for projecting n laser light beamswhich emanate from an array of n laser diodes. Light source 10 iscomposed of an individually drivable array of n single stripe laserdiodes 12. It is customary for a light source of this kind to have up to100 single stripe laser diodes, advantageously between 10 and 60. Thesingle stripe laser diodes have emitter surfaces of a typical size of1×5 micrometers², and emitted laser radiation of an advantageous beamquality with a low diffraction index M². The individual laser diodes areusually spaced apart on the array by distances of between 100 and 1000micrometers. Imaging optics 16 is used to project the n laser beams onton image points 110 on a plate 18. Printing plate 18 is advantageouslysituated in the foci of laser beams 14. It is of particular benefit thatimaging optics 16 not only modifies the diameter proportions of thelaser beams (perpendicularly and in parallel to the axis defined by then points), but that it also corrects the distance by which the imagepoints are set apart from one another. In other words, both the spotsize of n image points 110, as well as their position relative to oneanother and their spatial interval are adjustable. As a general rule,the individual laser diodes are spaced apart by a constant distance;however, the minimum requirement for an advantageous imaging is onlythat spatial interval l of n image points 110 be constant. The spatialinterval l among the n image points is greater than the pitch distance pamong the dots.

Light source 10 can be used in continuous operation. To produceindividual light packets, the laser emission is suppressed accordinglyby a specific time interval. One specific embodiment also provides,however, for employing a light source 10 which emits pulsed radiation.When working with pulsed radiation, the repetition rate of the lightpulses must be at least exactly as great as the pulse frequency used togenerate the individual dots, so that at least one laser pulse isavailable for one dot. Imaging optics 16 can have reflective,transmittive, refractive, or similar optical components. These arepreferably micro-optical components. Imaging optics 16 can be bothenlarging as well as reducing, and also have different imaging scales inboth directions, in parallel and perpendicular to the active zone of thelasers. This is particularly beneficial for correcting divergence andaberration. The physical or chemical properties of the surface ofprinting plate 18 are modified by the laser radiation. Printing plateswhich are erasable or rewritable are advantageously used.

In one preferred specific embodiment, light source 10 is disposed on acooling element 112. Light source 10 is linked via a current-supply andcontrol line 114 to control unit 116. Control unit 116 has individualcomponents, which enable individual laser diodes of the array to bedriven or regulated separately from one another. Cooling element 112 islinked via a line 118 for controlling cooling element 112 to temperaturecontrol 120.

A detector 122 is provided to test for correct functioning and determinethe power output of individual laser diodes 12. The design of thedetector can be such that an individual measuring device is provided foreach laser diode or, however, that a measuring device checks theindividual laser diodes at the time of replacement or as needed.Detector 122 is advantageously linked via connection 124 to control unit116, so that the power output is processed, inter alia, as a parameterfor generating a control signal in laser control 116.

A device of this kind in accordance with the present invention can beprovided as an internal device in a print unit or a printing press, orbe provided externally thereto.

FIG. 2 illustrates the imaging of a printing plate, which is situated ona rotatable cylinder. Light source 20 produces n laser beams 22, whichare projected using imaging optics 24 onto n image points 210. The nimage points are uniformly spaced apart and are disposed on one axis.Printing plate 28 is situated on a cylinder 26, which is rotatable aboutits axis of symmetry 25. This rotation is denoted by arrow B. Lightsource 20 can be moved in parallel to axis of symmetry 25 of thecylinder on a linear path shown by double arrow A. For continuousimaging, cylinder 26 rotates with printing plate 28 in accordance withrotational motion B, and the translation of the light source along thecylinder is in accordance with moving direction A. The feed rate isdetermined by the number of laser beams 22 and the width p of a dot. Theresult is an imaging which encircles axis of symmetry 25 of cylinder 26on a helical path. The path of image points 210 is indicated by lines212. In other words, once imaging of n points is complete, a relativedisplacement of printing plate 28 and image points 210 follows with avector component perpendicular to the direction defined by the line ofthe n image points, by a first specific amount, so that n points areexposed again at another position of printing plate 28. In this manner,so-called raster scan lines of image points are formed. For eachspecific spacing of adjacent raster scan lines and number n of imagepoints, a second specific amount of a necessary displacement is derived,in parallel to the axis defined by the line of n image points, so that acontinuous imaging, i.e., the imaging of each raster point provided onprinting plate 28, is possible using the interleaving raster scan linemethod.

In one alternative exemplary embodiment, image points 210 can also bemoved in a meander shape over printing plate 28, in that a completeimaging is initially carried out along one line in parallel to axis ofsymmetry 25 of cylinder 26, and a step-by-step rotation is subsequentlycarried out about axis of symmetry 25 of cylinder 26.

It is clear that it is only a question of a relative movement betweenimage points 210 and printing plate 28. This relative movement can alsobe achieved by a movement of impression cylinder 26. For both movingdirections of translation A and rotation B, it holds that the movementscan take place continuously or step-by-step.

In another alternative specific embodiment, the device for imagingprinting plates having light source 20, imaging optics 24 and the like,can also be provided inside of impression cylinder 26, thereby achievinga space-saving configuration.

Prior to describing the interleaving raster scan line method in detailon the basis of a figure, general explanations in this regard areprovided. As already mentioned, to image a printing plate, the imagepoints are shifted over the printing surface, initially with a componentperpendicular to the direction defined by the line of the image points,in order to form so-called raster scan lines. A contiguous line of dotsis understood to be a line formed by the subsequent displacement in thedirection defined by the direction of the dots. In other words, the dotsare situated at the same level and belong to different scan lines thatare scanned next to one another.

The distances between the n image points simultaneously produced by theindividual n laser diodes are selected to be constant; the lengthbetween two adjacent image points l is advantageously an integralmultiple m of pitch distance p between the dots, in other words l=m×p. Acontinuous inscription, i.e., each raster point is exposed at least onceto the image point of a laser, with n simultaneously scanned imagepoints at the distance l=m×p, m being a natural number and p thedistance between the dots, is always possible if one selects anappropriate displacement. The width of the displacement isadvantageously equal to the number of image points.

It can also happen, in this context, that one point is inscribed severaltimes. A continuous inscription, in other words each dot is exposedexactly once, is especially possible when the number of image points nand their spatial interval l, measured in units of the pitch distance pof the dots, do not have a common divisor. Expressed differently, n andm are prime. This is the case, for example, when m and n are differentprime numbers. At the same time, the displacement, which is stipulatedby the direction given by the line defined by the n image points, is tobe selected as n. In the process, an edge area of the size r:r=n×m−(n+m−1) results at the beginning and end of the line to bescanned.

Since each of the laser diodes can be driven discretely, each dot can beconfigured individually. The performance of one specific laser beamprovided for inscribing a raster point is stipulated in accordance withthe image data information given. This enables the optical density ofdifferent dots to be achieved on an individual basis.

FIG. 3 illustrates the interleaving raster scan line method forinscribing printing plates, based on an example of five image points,which are produced at the same time by the simultaneous irradiationusing five individual laser diodes. In this figure, dots are depicted insimplified form as small boxes. As already mentioned, each dot must beexposed once to an image point of a laser, so that it is exposed inaccordance with the image data given, or it can be left unchanged. Inthis example, a contiguous line to be scanned is composed of dotsdisposed side-by-side and without gaps in a row. Their pitch distance isdesignated by p. In FIG. 3, the group of simultaneously exposed dots 30is composed of five image points exhibiting a uniform spatial intervall. In first imaging 32, five unit points are exposed with spatialinterval l=3p. The group of simultaneously produced dots 30 is thendisplaced by five unit points, since, in this example, five dots aresimultaneously written in the direction defined by the axis of the dots;in this example, to the right. In second imaging step 34, five imagepoints are again set. A renewed displacement by five unit points followsiteratively in the direction defined by the axis of the dots; in thisexample, to the right. In imaging step 36 that follows, five points areset once again. From this sequence, it is apparent that the printingplate can be inscribed without gaps: each dot represented by a small boxis exposed once to the image point of a laser. In each renewed imagingfollowing a displacement step by five units of length, measured in unitsof p to the right, the same pattern is always produced at alreadyexposed and still unexposed dots, as is apparent in 38. In other words,at its right end, the line of exposed image points still has certaingaps of unexposed raster points. If, at this point, a further imaging offive raster points takes place at the right end, then the same sequenceof still unexposed and already exposed raster points is obtained. At thesame time, the portion of the line composed of completely inscribed dotsbecomes longer and longer. Likewise apparent in 38 is the edge area ofsize r, in this case eight dots, measured in units of pitch distance pof the dots.

Even in the event that individual single stripe laser diodes in thearray fail, the proposed interleaving raster scan line method can beused for scanning. Particularly when the number of n image points of thelaser beams and the spatial interval between two adjacent image pointsl, measured in units of p, are prime, the imaging speed is at a maximum.In other words, it is possible to specify an increment, so that eachpoint to be scanned is exposed only once to an image point of the laserbeams.

In the event that one or more of the single stripe laser diodes in thegroup of simultaneously scanned image points 30 is not functioningproperly, it is still possible to inscribe using the interleaving rasterscan line method. In such an instance, it is always the largest sectionof the group having equidistant, adjacent image points that is used forinscription. In order to achieve a continuous inscription, one must thenobviously reduce the increment. It is beneficial to do so in accordancewith the above established rules with respect to the properties ofnatural numbers.

The interleaving raster scan line method can be used to image a printingplate for every combination of distances between adjacent image points land their number n. To continuously inscribe the printing plate,however, one must select appropriate parameters. If one image pointshould drop out, an imaging at a reduced speed is possible.

The described interleaving raster scan line method requires amultiplicity of laser beams to inscribe a printing plate. These can alsobe produced by laser light sources other than the advantageously usedlaser diodes. To modify the projected distance between the individuallight sources, one advantageous refinement provides for tilting theprinting plate by an angle that differs from zero with respect to theplane disposed perpendicularly to the n laser beams.

Another advantageous refinement of the present invention provides for atwo-dimensional array of n₁×n₂ image points. When extrapolatingaccordingly, from one to two dimensions, spatial intervals l₁ and l₂between adjacent points in the two mutually perpendicular directionsmust be constant in each case, so that n₂ lines can be processed inparallel at distance l₂, in accordance with the discussedone-dimensional interleaving raster scan line method employing n₁ imagepoints at distance l₁. A displacement is then likewise carried out inthe perpendicular direction in accordance with the rules established forthe interleaving raster scan line method, in order to densely placedots.

Printing plates as defined wherein can include all types of printingforms.

Reference Symbol List

10 light source, individually drivable laser diode array

12 single stripe laser diodes

14 light beam

16 imaging optics

18 printing plate

110 image point

112 cooling element

114 current-supply and control line

116 control unit

118 temperature-control line

120 temperature control

122 detector for testing functioning and measuring power

124 connection to control

20 light source

22 laser beams

24 imaging optics

25 axis of symmetry

26 cylinder

28 printing plate

210 image points

212 path of image points

A translation

B rotation

30 group of simultaneously scanned dots

32 first imaging

34 second imaging

36 third imaging

38 iterative imaging

l spatial interval of the image points

p pitch distance of the dots

n number of image points

r edge area

What is claimed is:
 1. A device for imaging printing plates comprising:an array of n laser diodes which image n image points, so that one laserdiode of the array is allocated to each i-th point, with i being from{1, . . . , n}, the n image points being separated by a spatial intervall between adjacent image points, with a pitch distance p of dots to beimaged by the array, the laser diodes being individually-drivable singlestripe laser diodes, wherein the spatial interval l between adjacentimage points, measured in units of the pitch distance p of the dots, isan integral multiple m of the pitch distance p between the dots; andwherein the integral multiple m and the number n of image points have nocommon divisor; and at least one detector for testing for correctfunctioning and determining a power output of one or of a plurality ofthe laser diodes.
 2. The device as recited in claim 1 wherein thespatial interval l of adjacent image points, measured in units of thepitch distance p of the dots, is smaller than the number n of the imagepoints.
 3. The device as recited in claim 1 wherein the multiple m andthe number n of the image points are prime numbers.
 4. The device asrecited in claim 1 further comprising imaging optics for correcting atleast one of divergence and aberration.
 5. The device as recited inclaim 1 further comprising a control unit, at least one of the laserdiodes of the array being controlled by the control unit.
 6. The deviceas recited in claim 1 wherein the number of laser diodes in the array isbetween 10 and
 100. 7. The device as recited in claim 1 furthercomprising a detector for determining a power output of at least one ofthe plurality of laser diodes and a laser controller, the lasercontroller being controlled as a function of the power output determinedby the detector.
 8. The device as recited in claim 1 wherein at leastone laser diode is a pulse controlled laser.
 9. The device as recited inclaim 1 wherein a repetition rate of the light pulses is at leastexactly as great as a pulse frequency of the pulse-controlled laser inorder to displace the individual dots.
 10. The device as recited inclaim 1 further comprising imaging optics including at least onereflective optical element.
 11. The device as recited in claim 1 furtherincluding imaging optics having micro-optical components.
 12. The deviceas recited in claim 1 wherein the device is in a computer-to-plate unitand the printing plate is an offset printing plate.
 13. The device asrecited in claim 1 wherein the device further comprises a cylinder, theprinting plate being situated on the cylinder.
 14. The device as recitedin claim 1 wherein the array of laser diodes is movable with respect tothe printing plate.
 15. A device for imaging printing plates comprising:an array of n laser diodes which image n image points, so that one laserdiode of the array is allocated to each i-th point, with i being from{1, . . . , n}, the n image points being separated by a spatial intervall between adjacent image points, with a pitch distance p of dots to beimaged by the array, the laser diodes being individually-drivable singlestripe laser diodes, wherein the laser diodes are spaced apart on thearray by a distance of between 100 and 1000 micrometers, and a width ofemitter surfaces of the laser diodes is less than 10 micrometers. 16.The device as recited in claim 15 wherein the width is 5 micrometers.17. An interleaving raster scan line method for imaging printing platesby generating raster points using an array of n laser light sources,which use an imaging optics to image n image points arranged on a line,the n image points being separated by a spatial interval of adjacentpoints l, comprising the steps of: simultaneously generating n imagepoints on a printing plate by a plurality of laser light sources;generating a relative motion between the image points and printingplate; displacing the image points with a translation componentperpendicular to the line of the image points by a first specificamount; displacing the n image points in a direction defined by the lineof the n image points by a second specific amount; repeating thedisplacement steps, an amount of the second specific displacement beinggreater than the spatial interval l of adjacent image points.
 18. Theinterleaving raster scan line method as recited in claim 17 wherein thesecond specific amount, measured in units of the pitch distance p ofdots to be imaged, is equal to the number n of image points.
 19. Theinterleaving raster scan line method as recited in claim 18 wherein thespatial interval l of the image points is an integral multiple of thepitch distance p of dots of the laser diodes.
 20. The interleavingraster scan line method as recited in claim 17 wherein the spatialinterval l of the image points, measured in units of a pitch distance pof dots of the laser diodes, and the number of laser diodes n have nocommon divisor.
 21. The interleaving raster scan line method as recitedin claim 20 wherein the spatial interval l of the image points, measuredin units of the pitch distance p of the dots, and the number of laserdiodes are prime numbers.
 22. A print unit comprising at least onedevice for imaging printing plates, the device including an array of nlaser diodes which image n image points, so that one laser diode of thearray is allocated to each i-th point, with i being from {1, . . . , n},the n image points being separated by a spatial interval l betweenadjacent image points, with a pitch distance p of dots to be imaged bythe array, the laser diodes being individually-drivable single stripelaser diodes; the spatial interval l between adjacent image points,measured in units of the pitch distance p of the dots, being an integralmultiple m of the pitch distance p between the dots; wherein theintegral multiple m and the number n of image points have no commondivisor; and at least one detector for testing for correct functioningand determining a power output of one or of a plurality of the laserdiodes.
 23. A printing press comprising at least one print unit inaccordance with claim
 22. 24. A print unit comprising at least onedevice for imaging printing plates, the device including an array of nlaser diodes which image n image points, so that one laser diode of thearray is allocated to each i-th point, with i being from {1, . . . , n},the n image points being separated by a spatial interval l betweenadjacent image points, with a pitch distance p of dots to be imaged bythe array, the laser diodes being individually-drivable single stripelaser diodes, the laser diodes being spaced apart on the array by adistance of between 100 and 1000 micrometers, and a width of emittersurfaces of the laser diodes being less than 10 micrometers.
 25. Aprinting press comprising at least one print unit in accordance withclaim 24.